The development of medical delivery devices has become more and more directed towards the ability for the patient themselves to administer a medicament in an easy, safe and reliable way. For these medical devices, there are generally different types of medicaments that can be stored for a long time and that are packaged in cartridges or the like, containing a ready-to-use medicament in liquid state. There are also other type of medicaments that are a mixture of a medicament agent (e.g. lyophilized, powdered or concentrated liquid) and a diluent (e.g. water, dextrox solution or saline solution), wherein these type of medicaments can not be pre-mixed and stored for a long time because the medicament agent is unstable and can be degraded and loses its effect quickly. Hence, the patients have to perform the mixing within a limited time period prior to the delivery. In order to facilitate the mixing, cartridges or the like comprises at least two chambers, one chamber containing the medicament agent and the other chamber containing the diluent.
Conventionally injection devices are elongated devices with proximal and distal ends. Moreover, said injection devices comprise a cartridge or the like, with one or more chambers containing a medicament to be delivered. Said conventional injection devices are further provided with a plunger rod that is adapted to be in contact with a piston provided inside the cartridge or the like. When a patient or operator wants to administer a dose of medicament, a pointed needle is mounted on the distal end. Upon a force exerted on the piston by the plunger rod, the piston will move forward inside the cartridge to expel the medicament from the cartridge or the like. In many cases, the cartridge contains some quantities of air because it is difficult to obtain a complete filling of medicament.
With a multi-chamber cartridge the medicament agent has to be mixed with the diluent. A multi-chamber cartridge or the like usually contains quite a lot of air, especially when the medicament agent is in powder form, which air is present when the medicament and the diluent are mixed. Further, also the needle contains air in its interior when it is mounted. It is also known that air can be sucked into the cartridge if the patient lets the needle be mounted on the cartridge or the like for a longer period of time and atmospheric conditions change.
This entrapped air is often desirable to get removed before injection. One danger is otherwise that the entrapped air is injected into the patient, which could cause injuries. Another aspect, even if the air would not cause direct harm, is that the first injected dose will be smaller than intended because of the amount of air, hence dose accuracy is reduced. There is further a psychological aspect. A patient may be worried if there is air entrapped and visual in a cartridge of an injector, even though the air may not cause any harm. The mere knowledge that there is air in the cartridge, and that the air might be injected, could be sufficient for a patient not to do want to use the device.
In order to remove the air or gas that is entrapped in the cartridge or the like and/or the needle, priming needs to be performed by means of expelling a small dose of medicament. If at least a droplet or a short liquid jet is not expelled at the sharp point of the needle, another small dose of medicament should often be actuated. In view of that, sometimes the patients desire to set an unnecessary large dose in order to be sure that all air has been expelled. Further, some cartridges have a very small overfill, i.e. the excess quantity of medicament apart from the actual doses. It could also be that the medicament is very expensive. In both cases it is necessary to limit the amount of medicament that is expelled during priming.
Patent document EP 1 365 823 discloses a needle-less injector having a dual chamber medicament cartridge. When a front nozzle portion of the injector is threaded into the injector body, a drug plunger is forced into the medicament cartridge for expelling any air in the medicament cartridge. When the nozzle portion reaches its stop position a drive gas cartridge is activated for forcing medicament through the skin of the patient.
The drawback with this solution is that the plunger of the device has to be manipulated, rendering the construction of the device unnecessarily complicated for the function and purpose, which solution also is very linked to needleless injectors.
The document US 2004/0236285 discloses a medication dispensing apparatus comprising a priming mechanism accessible from the outside of the apparatus in order to prime the medicament container. The priming can be performed whether or not the apparatus is in a loaded state or not. The priming mechanism comprises a drum part that is rotatable manually by a user. The drum can only be rotated in one direction, the priming direction in order to ensure that the user performs the priming correctly. However, the priming mechanism can not be locked in the priming direction, and is readily accessible by any user, whereby there is a pronounced risk that priming operations may be performed intentionally or unintentionally during subsequent injections of doses, whereby the precision and possible a number of doses are lost.